This invention relates in general to seals and more particularly to a seal that is capable of venting the space isolated by it and to a sealed bearing in which the seal is installed.
The bearings on which the wheels of large over-the-road trucks and trailers revolve are housed within cavities that are isolated at their ends by lip seals. The typical lip seal has a case that is normally pressed into the wheel hub and an elastomeric lip that bears against a wear surface along an axle or spindle. The lip is somewhat delicate and to protect it from damage and to further orient it the most efficient position for sealing, it is turned axially inwardly into the isolated cavity. Thus, a significant portion of the lip is exposed to the cavity, and in many seals a garter spring encircles the exposed portion of the lip to insure that the lip remains in contact with the wear surface.
Sometimes the inwardly directed lip seals too effectively, and as a result the entire seal may be damaged. In this regard, truck brakes are located next to and indeed are mounted on the wheel hubs. Braking generates a considerable amount of heat, and if the braking is sustained, the heat will soak back into the hub and elevate the temperature in the bearing cavity. This in turn elevates the pressure within the cavity, and the elevated pressure forces the seal lip against the wear surface with even greater force. Thus, excessive friction may develop between the seal lip and wear surface, causing deterioration of the seal lip.
Since conventional lip seals react adversely to elevated pressures, they do not lend themselves to simple expedients for relubricating the bearings that they isolate, that is conventional lubrication fittings, for the lubricant must be forced through these fittings under considerable pressure. While it is desirable to relubricate wheel bearings from time to time, the advantages afforded by having simple lubrication fittings in the hubs, are more than offset by the damage that will result if the bearings in the hub receive excessive lubrication. In particular, without the incorporation of a venting means, the introduction of a lubricant through a fitting and into a bearing will increase the pressure within that bearing and of course cause the seal lips at the ends of the bearing cavity to bear against their respective wear surfaces with substantial force. Indeed, the pressure may become great enough to invert the seal lips or dislodge an entire seal from the hub. Instead of providing lubrication and vent fittings for quickly lubricating the bearings, the usual practice is to repack the bearings with grease about every 20,000 to 30,000 miles by disassembling the hub. Wheel bearings that operate in oil run longer, although even they are disassembled, cleaned, and inspected at about every 150,000 miles, to thereupon be reassembled with the proper amount of oil.
The purchase of a truck or trailer involves the expenditure of a significant amount of capital, and whenever a truck or trailer is laid up for bearing maintenance, this capital is not to be put to effective use. Even so, it costs considerably less in terms of direct labor and down time to relubricate bearings using conventional procedures than it does to replace a bearing damaged by the absence of adequate lubrication.